1. Field of the Invention
The present invention relates generally to alkali metal thermal to electric converter (AMTEC) cells. More specifically the present invention relates to AMTEC cells which operate in a liquid anode condition.
2. Description of the Related Art
An AMTEC cell generally comprises a closed container separated into high- and low-pressure regions by a solid electrolyte. In the higher pressure region, alkali metal is in thermal contact with a heat source. In the lower pressure region, alkali metal is condensed by the removal of heat. During operation of the AMTEC cell, a heat source raises the temperature of the liquid alkali metal within the high-pressure zone to a high temperature and a correspondingly high vapor pressure which creates a vapor pressure differential across the solid electrolyte. The resulting electrochemical potential difference between the regions causes migration of the alkali metal ions into the solid electrolyte with concomitant loss of electrons. These electrons flow through the external circuit and recombine with alkali metal ions passing out of the solid electrolyte at a porous electrode, neutralizing the alkali metal ions. In this way, the cell acts as a source of electrical potential for an electrical circuit. AMTEC cells deliver their electrical power to output terminals connected to positive and negative leads. The positive lead is connected to the cathode and the negative lead is usually connected to the metallic cell wall. A plurality of solid electrolyte structures may be connected in series within an AMTEC cell to increase the voltage of an AMTEC cell.
Most AMTEC cells employ at least one solid electrolyte structure in the form of a beta-alumina solid electrolyte (BASE) tube of varying geometries with the high-pressure alkali metal exposed to the BASE tube inner surface, and low-pressure alkali metal exposed to the BASE tube outer surface. The BASE tube element's inner and outer surfaces are overlaid with permeable electrodes, which are connected to each other through an external load circuit. The BASE tube provides the functions of the solid electrolyte structure discussed previously. Neutral alkali metal atoms incident on the BASE tube's inner surface give up their electrons at the anode (in this case the inner electrode). The resulting sodium ions pass through the tube wall under the applied chemical activity gradient, and the emerging alkali metal ions are neutralized at the cathode (in this case the outer electrode) by electrons returning from an external load. AMTEC cells deliver electrical power to an external load via two output terminals, a positive lead connected electrically to the cathode (outer electrode), and a negative lead, frequently connected electrically to the metallic BASE tube wall.
The neutral alkali metal vapor leaving the outer surface of the cathode electrode flows through the condenser space until it condenses at a low temperature condenser surface. The condenser transports the alkali metal condensate to an artery. From there, the alkali metal condensate flows through the artery containing a fine pore membrane consisting of a packed matrix of metal, ceramic wires, tubes or particles, similar in function to those used in heat pipe wicks. The liquid alkali metal evaporates at a high-pressure at the surface of an evaporator wick that is coupled to the artery membrane. The high-pressure alkali metal vapor is returned to the inside of the BASE tubes through a plenum at the heated end of the AMTEC cell.
Present state of the art AMTEC systems use single or multiple BASE tubes having a dry inner surface or anode. The inner surface of the present state of the art BASE tubes is maintained in a dry state due to the BASE tube's operating conditions which are designed to prevent condensation of alkali metal vapor on the BASE tube's inner surface. Liquid alkali metals are very good conductors of electrical current and create electrical shorting problems when present on the inner surface of a BASE tube. In the context of this invention description, the term "ground" is to be understood as referring to a common metallic conductor which may or may not be at the same electrical potential as the cell wall or tube support plate. The term "grounded" is to be understood as referring to the existence of a low resistance electrical connection to such a "ground". The present state of the art BASE tube is equipped to deal only with alkali metal vapor, since any liquid alkali metal present on the inner anode has the potential to short circuit to ground the inner anode by flowing into electrical contact with the tube support plate or other grounded structure in the AMTEC cell. These considerations have led to the constraint that state of the art BASE tube designs have the alkali metal on the anode side of the electrolyte be maintained only as a vapor.
Although the present design of BASE tubes makes them well suited for many applications, operating the BASE tubes with a dry or vapor anode has several disadvantages such as high contact impedance, lowered BASE power density, and the need for additional parts, adding assembly complexity. More specifically, the inner surface electrode used in state of the art vapor anode cells is a thin, sodium-permeable, metallic material applied to the inside surface of the BASE tube and brought into contact with a current collection grid. This arrangement results in less than ideal electrode performance because of the significant electrical contact resistance between the current collection grid and inner electrode on the BASE surface. The additional anode current collection grid used by a BASE tube operating with a vapor anode also leads to an increase in the number of parts and complexity of an AMTEC cell.
As mentioned previously, liquid alkali metal present on the inner surface of a BASE tube designed for operation with alkali metal vapor can cause significant electrical problems. However, the electrical characteristics of alkali metal which make it undesirable for a BASE tube operating with a dry or vapor anode can be very desirable for a BASE tube assembly designed for operation with a liquid anode. The low contact impedance, high thermal conductivity, and high latent heat of vaporization of liquid alkali metals are characteristics which may be taken advantage of by a BASE tube operating with a liquid anode, so long as the previously mentioned electrical shorting problems are overcome.